1. Field of the Invention
This invention relates to the field of processes for creating metallization patterns on the surfaces of a substrate and more particularly to processes for creating high resolution metallization patterns on the surfaces of quartz crystals, hybrid circuits or semiconductor devices using photolithographic techniques which are readily adapted to automation.
2. Description of the Prior Art
The normally accepted manufacturing process for producing metallization areas on quartz crystal products is a stenciling process called shadow masking. In this process an apertured mask is placed in contact with a crystal blank which is sometimes polished. The apertures in the mask form a stencil pattern corresponding to places where metallization is desired and will be deposited. The masked blank is placed in a suitable vacuum cleaner. Metal is then evaporated within the chamber and adheres to the crystal's surface in the places exposed by the apertures of the stencil. In such a process resolution is moderate at best with dimensional accuracy being only within approximately 1/1000th inch. The resolution is limited by the accuracy with which a shadow mask can be manufactured by various machining or etching techniques.
If metal is to be deposited on both surfaces of the crystal by shadow masking, alignment of the desired patterns from front to back is difficult to control. This alignment typically varies from one crystal to the next, resulting in inconsistency in the crystal's motional parameters. The crystal's motional parameters determine the center frequency, passband shape, spurious response, and other electrical parameters. The masks are typically made of stainless steel or other metal with thickness of about 3/1000th of an inch. Intricate stencil patterns in this thin metal are easily bent or otherwise damaged.
In addition, due to the stenciling nature of the masks, not all patterns are producible by this method without using two or more iterations of the process. This generally results from the masks being punched or etched from a single sheet of metal. Many types of metallization patterns are impossible to fabricate. In addition, such masks are difficult and time consuming to make. This is a serious drawback to the experimental stages of crystal development.
In one method for making a tape carrier for manufacturing leads for integrated circuits, an adhesive backed flexible insulating tape is used to carry thin metal sheets of foil on its surface to produce integrated circuit leads. The insulating tape has one entire surface covered with adhesive and has holes punched in its surface which allow access to both sides of portions of the copper foil for processing by photolithographic techniques. Only that portion of the foil exposed by the aperture in the tape carrier is available for photolithographic processing on the surface contacting the adhesive. Bonding of the foil to the tape is accomplished by the adhesive properties of the tape.
Care must be taken to ensure that the finished assembly is not exposed to high temperatures which would cause the adhesive to lose strength. Care must also be exercised in the selection of developing and etching agents used in the processing of the leads also to ensure that they do not react adversely with the adhesive. The adhesive is prone to attracting dirt and dust wich can contaminate the chemicals used in further processing the tape carrier assembly.
Although other prior art devices are useful for forming flexible metal leads they are not well suited for processing more brittle workpieces such as ceramic or quartz crystal substrates. It is evident that separating such workpieces from the adhesive surface of a tape carrier would likely result in damage to a large number of the substrates. This would make production yield low and assembly costs unreasonably high. A further drawback of the tape carrier is that it is obviously usable only once as attempted reuse would degrade the adhesive properties of the tape carrier rendering the adhesive unreliable.
It is also known that both sides of a silicon wafer may be exposed simultaneously using double sided photolithography. Such exposures are normally made using an "alligator mask," which holds photolithographic masks in direct contact with photoresist coated silicon wafers. Other methods of double sided photolithography are known in the art.